1. Field of the Invention
This invention is directed to circuit breakers and in particular to circuit breakers having mechanisms which prevent movement of the circuit breaker operating handle to the off position if the contacts are welded closed.
2. Background Information
A common type of circuit breaker has a fixed electrical contact, and a movable electrical contact mounted on a movable contact arm. The contacts are closed and opened by rotating a handle between an on and off position, respectively. A latchable cradle connected to the movable contact arm by a spring operated toggle device is held in a latched position by a trip device. In response to predetermined overload conditions in the circuit breaker, the trip mechanism unlatches the latchable cradle and the spring operated toggle device rotates the movable contact arm to open the contacts. When the circuit breaker is tripped in this manner, the spring operated toggle device also moves the handle to a trip position intermediate the on and off positions. Thus, the handle provides a visual indication of the state of the circuit breaker. The circuit breaker is reset by moving the handle slightly past the off position to relatch the latchable cradle, and then to the on position to reclose the contacts.
It is possible under some overload conditions for the contacts of a circuit breaker to become welded closed. Under these circumstances, the trip device of the circuit breaker described above can respond to the overload condition by unlatching the latchable cradle. However, if the weld is of sufficient strength, the contact arm cannot be rotated and the contacts remain closed. Still, it may be possible to rotate the handle to the off position. Furthermore, in some installations, the circuit breaker is operated by a motor operator or a mechanical linkage which can force the operating handle to the off position even though the contacts are welded closed. Obviously, this is a very unsatisfactory situation.
Various remedies for the above condition have been proposed. Some of these solutions are not satisfactory because the operating mechanisms have a considerable number of moving parts, so that despite interlocks designed to prevent movement of the handle to the off position when the contacts are welded, the mechanism can be distorted or parts can even be bent to force the handle to the off position. In fact, some codes require that the device be able to withstand forces up to three times the normal operating force.
There is a need therefore for an improved positive off mechanism for a circuit breaker which is rigid enough to block movement of the operating handle to the off position despite the application of a force to the operating handle which is in excess of the normal operating force required to operate the circuit breaker.